Ordered arrays of nanoscale holes in metal films exhibit extraordinary optical transmission at resonant wavelengths [1,2]. Surface plasmons (SPs) are oscillating electromagnetic waves at the interface of a metal and a dielectric. Due to the confinement of the electromagnetic wave near the surface [3,4], surface plasmon resonance (SPR) has found many applications in biosensing [5,6]. In general, biosensing involves the detection of the adsorption of a biomarker to a functionalized surface [7]. In the case of SPR, the molecular adsorption may be detected directly, without a label, presenting several advantages as reported previously [8].
Incorporation of nanohole arrays in microfluidic platforms has been reported previously [9-11]. A flow-through nanohole array based sensing platform has been recently proposed [12] with several advantages over previous reported sensing schemes, such as enhanced transport of reactants via nanoconfinement and solution sieving. The use of this flow-through sensing scheme has been limited to its inherent sensing purpose with the benefits mentioned before. However, under an externally applied electric field, the metallic nature of the plasmonic sensor, comprising metallic and insulator layers, creates a localized potential gradient at the liquid-metal interface which influences the electrohydrodynamic transport of both the charged species molecules and the bulk fluid. In established electrohydrodynamic-driven microfluidic systems, the fluid may be transported by electroosmosis (EO) due to the preferential ionic distribution in the electric double layer (EDL). Charged molecules in the system are transported by electrophoresis (EP) which can oppose the EO transport of the bulk fluid depending on the molecule charge sign. Recently, Hlushkou et al. [13, 14] reported an electric field gradient focusing technique using a straight microfluidic channel with an embedded floating gold electrode. The electrode in this case, is placed normal to the electric field lines from the external circuit, resulting in the concentration of species due to the formation of sharp electric field gradients in the microfluidic channel. However, a metallic element placed in series with the microfluidic system, like the case of a nanohole array based sensor, will conduct the current and will drop the local electric field within the metal region. This will cause a null contribution of the EO and EP driving forces on the bulk and the charged species on the metallic portion of the sensor, where the fluid is transported only by the advective force from the EO flow in the microfluidic channel. On the other hand, the local electric field strength in the nitride portion of the sensor will be higher and will produce a higher EP contribution. The species may then become quasi-stationary and, therefore, locally concentrated inside the nanoholes.
Unlike the prior art, where nanoholes are used as passive elements only for sensing, the present invention disclose the use of nanohole arrays as active elements for achieving both concentration and sensing of a desired species.
The present invention is directed to apparatus and methods of operation that are further described in the next Brief Description of The Drawings, the Detailed Description of the Invention, and the claims. Other characteristics and advantages of the present invention will become evident from the following detailed description of the invention made with reference to the accompanying drawings.
According to representative examples, apparatus comprise a voltage source coupled to apply a voltage to a substrate having a nanohole array, wherein application of a voltage is operative to concentrate and to sense charged species contained in a surrounding fluid. In typical examples, the nanohole array comprises a plurality of through holes defined in a substrate that comprises two or more layers. A microfluidic channel is fluidically coupled to the nanohole array. In some examples, the substrate with the nanohole array is placed so as to divide the microfluidic channel into two portions, wherein the two portions are fluidically coupled by the nanoholes. Two electrodes are provided that are coupled to the voltage source, wherein at least a portion of each of the two electrodes is situated in the microfluidic channel so as to apply a voltage from the voltage source to the fluid. In some examples, the first layer is made of a metal or a semiconductor material, and the second layer is made of an insulator. In other examples, the substrate comprises a plurality of layers, wherein at least one layer is made of a metal and at least one layer is made of an insulator. In other alternatives, a plurality of nanohole arrays is coupled to the voltage source so as to be operative to concentrate and to sense charged species, molecules or particles contained in the surrounding fluid. In particular examples, the nanoholes are elliptical, rectangular, biaxial, oval, or round, and the nanoholes have diameters of between 1 nm and 1000 nm. According to representative examples, a period of the nanohole array is equal to or greater than a nanohole diameter. In representative embodiments, the thickness of the metallic layer and the insulator layer are between 1 nm and 1000 nm. In a particular embodiment, the metal layer is a gold film and the insulator layer is a silicon-based material such as silicon nitride. The gold film can be formed by a thermal process such as evaporation or an electrical process such as electro-deposition, or otherwise formed.
In some examples, the thickness of the gold layer is approximately 100 nm and the thickness of the silicon nitride layer is approximately 100 nm. In other examples, the substrate comprises a third layer situated between the gold layer and the silicon nitride layer, wherein the gold layer and the silicon nitride layer are adhered to the third layer. The cross section of the microfluidic channel for fluidic access can be square, irregular, round, or rectangular. In some examples, the length of the microfluidic channel is between 1 μm and 20 cm. and the microfluidic channel is defined by a ceramic material or polymer material. Typically, the microfluidic channel is defined in a material that is transparent to light at wavelengths between 300 nm and 800 nm and the electrodes are made of a semiconductor material or a metal. The electrodes can be integrated with the microfluidic channel or be separate, and the microfluidic channel can be square or round. In some examples, a light source is situated and configured to direct an incident light flux to the nanohole array and the incident light flux consists of a single wavelength or a combination of two or more wavelengths. An optical detector is configured to receive light transmitted or reflected by the nanohole array.
Methods comprise coupling a fluid to a substrate having at least one through nanohole, concentrating an analyte at or in the nanohole by applying a voltage, and detecting the concentrated analyte. In some examples, the substrate comprises a plurality of through nanoholes that can be arranged in an array, randomly arranged, or otherwise arranged. The nanoholes can have a common diameter and/or a common axial length, or a distribution of nanohole sizes can be used. Typically, nanoholes have circular cross-sections but elliptical, arcuate, oblong, rectangular, polygonal, or other shapes can be used. A distribution of two or more nanohole shapes can be used, or a common shape used. The voltage can be applied to electrodes situated at least partially in the fluid, and the concentrated analyte detected by applying an optical flux to the nanoholes and detecting the transmitted or reflected optical flux. In some example, the applied optical flux is monochromatic, but two or more wavelengths can be used and detected individually or in combination.
The foregoing and other features and advantages of the invention will become more apparent from the following detailed description, which proceeds with reference to the accompanying figures.